Modern electrical devices are comprised of semiconductor circuits integrated into small packages, passive components, Printed Wiring Board (PWB) and solder. The complete assembly is often referred as a Printed Circuit Board (PCB) or Printed Circuit Assembly (PCA). The manufacture of a traditional PCA is a multistep process that may include several specialized and often expensive machines. These highly specialized machines are directed to one operation during the PCA manufacture. For example, a typical PWB, is manufactured using a thin sheet of copper foil that is laminated to a non-conductive substrate. The copper thickness may be 1.4 mils (1 ounce) and the substrate is typically FR-4 with a substrate thickness of 62 mils. Other thicknesses and substrates are also available.
Referring to FIG. 1A, conductive circuit elements or traces, such as lines, runs, pads and other wiring features, are created by removing copper from the laminated substrate by chemically etching or mechanically machining as illustrated in FIG. 1A. This subtiactive process leaves behind conductive traces 50, 50a, and 50b, located on a top surface of substrate 52. Referring to FIG. 1B, it may be necessary to include a second set of conductive traces, 50c and 54, which are electrically isolated from other conductive traces. In this case, conductive traces 50, 50a, and 50b, are etched to the top surface of the substrate 52 and conductive traces 50c and 54, are etched on a bottom surface of the substrate 52. Referring to FIG. 1C, by placing conductive traces, 50c and 54, on an opposite side of the substrate 52, the conductive traces, 50 and 54, can cross each other, for example at crossover 58, without making electrical connection.
Referring to FIGS. 1C and 2, when electrical connection is optionally used between the conductive traces, 50c and 50a, a via 60a is placed through substrate 52. The via 60a, often referred to as a “plated-through hole,” is typically manufactured in a two-step process wherein a hole is first drilled through the conductive traces, 50c and 50a, and the substrate 52 and then the hole is plated with copper thus making connection between the two conductive traces.
When complex circuits are manufactured especially for a small dimensional footprint, the complete board may contain multiple printed wiring boards stacked to allow copper lines to cross over each other while maintaining electrical isolation. Referring to FIG. 3, a four-layer PWB comprises substrate 61a and substrate 61b glued together with prepreg 62. PWB substrate 61a, has conductive traces 64 etched on a top side and conductive traces 65 etched to a bottom side. PWB substrate 61b has conductive traces 66 etched on a top side and conductive traces 67 etched to a bottom side. A via 70 is capable of connecting traces to any combination of conductive traces on different layers. The prepeg 62 is an insulating material used to electrically isolate conductive traces 65 and 66.
Highly specialized equipment is used to manufacture printed wiring boards in order to rapidly fabricate the boards at an economical cost. The etching equipment only performs one of several tasks optionally used to assemble a complete PCA. Once the printed wiring board is etched and drilled, the exposed copper traces are typically coated with solder, silver, nickel/gold, or some other anti-corrosion coating. The finished printed wiring board is then typically sent to another facility for assembly of electronic components onto the PWB. The attachment of electronic components, e.g., semiconductor and passive components, are trade using a solder reflow process. In one typical process, solder paste is applied to the PWB using screen printing techniques. Once the solder is printed onto the board, the electrical components are positioned onto the board. Positioning the components is often referred as “pick-and-place”. Components may be manually placed, often with tweezers, or in high volume production, components may be placed with a computer controlled machine. Once the components are all positioned on the solder paste, the PCA is placed in an oven to melt (reflow) the solder paste which will permanently attach the components to the board. Because of the multiple machines and technologies involved, this complete process can often take up to 4 weeks to complete.
The process of determining routing of the conductive traces is often performed using a Computer Aided Design (CAD) software tool. When using CAD, a user enters a schematic of a desired circuit including electrical components and package sizes. The CAD tool generates a set of files used as a mask when chemically etching each layer of the PWB. The same file is optionally used to control a Computer Numerically Controlled (CNC) milling machine when mechanically etching the PWB. When mechanically etching the PWB, the CNC milling machine removes copper along an outside edge of a desired conductive trace leaving behind a copper line that is electrically isolated from other conductive traces. The CAD tool output is in a file format that is typically Gerber. Gerber is an industry standard in the PWB industry which allows multiple vendors to share the same data without loss of information. The file format is optionally native to the CAD tool such as Eagle, OrCAD and Altium to name a few. In all cases, there is information for each layer of the PWB. During the layout process, the CAD tool will attempt to route the conductive traces based on a set of design rules which include the number of layers used in the PWB. For example, an entry in the CAD tool may be the use of a four-layer board which implies that there will be four independent layers of conductive traces. The CAD tool will route conductive traces to cross over each other while not making electrical contact. When the CAD tool knows that insulating layers exist between the multiple conductive layers and knowing that the insulating layers extend to the edges of the PWB, cross-overs are easily created by dropping the line from one layer of conductive traces to a second layer of conductive traces and moving across the layer and finally returning to the original side of the PWB. As an example, referring to FIGS. 1A-1C, and 2, conductive traces 50, 50a and 50b, are etched on the top side of the substrate 52 and it is desired to have conductive trace 50a make an electrical connection to conductive trace 50b. The conductive trace 50c is routed between the other conductive traces, 50a and 50b, and connection is made through a pair of vias, 60a and 60b, as the electrical connection is dropped to a lower conductive layer and runs underneath the conductive trace 50 through the conductive line 50c. When using vias, 60a and 60b, conductive pads 72 are typically optionally used around the hole location to compensate for tolerances when drilling the via hole. In FIGS. 1B and 1C, conductive pads 72 are etched on the top of the substrate 52. Conductive pads, 73 and 72, are etched on the bottom of the substrate 52 or any other lower level of a multilayer PWB. In practice, conductive pads, 70 and 72, typically have the same diameter however, this is not required. Connecting conductive trace 50a to conductive trace 50b is made through conductive pads, 72 and 73, and plated-through vias 60a and 60b. When the conductive trace routing is complete, the CAD tool will produce a drill file which includes the location of via hole 60a and via hole 60b. The drill file is used to control a CNC machine for drilling holes in the PWB. The drill file is included as an output from the CAD tool.
The conventional multilayer PCB production method is expensive and requires multiple machines to produce a multilayer PCB. Thus, a need exist for a single apparatus and method which can produce a completed circuit board and optionally populate the circuit board with components.